1. Field of the Invention
The present invention relates to an antenna for receiving or even transmitting telecommunication satellite beams.
The invention relates more particularly to an antenna with a single reflector having a wide field of view for receiving simultaneously a plurality of beams from geostationary broadcast satellites depointed by approximately 50.degree. from each other without using a motorized means for moving the reflector. The antenna is intended in particular for domestic installations in private houses, collective installations in buildings or community installations feeding cable network head ends for receiving a plurality of beams transmitted by radiocommunication satellites.
The antenna of the invention can also be used for professional applications such as data broadcast networks.
2. Description of the Prior Art
The individual satellite-beam receiving antenna for consumer use that is currently most widely used comprises a fixed reflector whose reflecting surface is a paraboloid of revolution which is circular with a diameter, or elliptical with major axis, from 50 cm to 90 cm. The axis of symmetry of the reflector is pointed toward the satellite. A receiver head is generally fixed by arms and positioned at the single focus of the reflector.
If the target satellite has an orbital position very close to other geostationary satellites, the antenna picks up the emissions from the various satellites by means of one or two receiver heads. However, if the user wishes to receive a plurality beams of satellites depointed by more than approximately 10.degree. the reflector must be turned and directed toward the chosen satellite either manually or by means of a motor. Thus this reflector type antenna cannot receive simultaneously from a plurality of satellites.
The antennas generally used for multisatellite reception have a reflector in the form of a parabolic or spherical torus. This type of reflector has a low efficiency, of 24% at most, because only a small part of the reflector is illuminated in any given direction. Consequently, the scanning capacities of a receiver primary source in front of this reflector can be increased only at the cost of a considerable increase in the surface area of the reflector.
U.S. Pat. No. 5,140,337 describes an antenna reflector with a high aperture efficiency which has a substantially cylindrical concave reflecting surface whose cross sections are deduced from two identical parabolas with axes tilted symmetrically relative to an azimuth plane. The article by William P. Craig, Carey M. Rappaport and Jeffrey S. Mason entitled "A High Aperture Efficiency, Wide-Angle Scanning Offset Reflector Antenna", IEEE Transactions on Antennas and Propagation, Vol. 41, No. 11, November 1993, pages 1481-1490, also concerns a reflecting surface of reflectors derived from two tilted and symmetrical parabolas, but in this case forming the section of a torus. U.S. Pat. No. 5,175,562 from the same inventor, Carey M. Rappaport, discloses an offset antenna of high efficiency ensuring a wide field of view, from -30.degree. to +30.degree.; the concave reflecting surface of the reflector of the antenna is deduced from two identical paraboloids with axes tilted symmetrically relative to the aiming axis of the antenna and is defined by a sixth order polynomial equation.
However, the geometry of the above reflectors is not satisfactory for individual reception because the focal length of these reflectors is too long. They require extremely directional receiver primary sources of large diameter, so increasing the overall size of the antenna, and the angular separation of radiation between consecutive beams is greater than 6.degree..
European patent application No. 0,700,118 discloses a continuous concave reflector reflecting surface which is deduced from a portion of a predetermined paraboloid by linear variation of the level of a point parallel to the axis of the paraboloid as a function of the wavelength.
This reflecting surface in practice produces relatively low gains for radiation directions depointed a few tens of degrees relative to the focus of the paraboloid.